Most oil fields have water saturated in the reservoir rock along with the oil. In every case the producing oil company has to deliver the oil to a certain sales specification. Free water is easily done with a vessel commonly called a Free Water Knockout (FWKO), however the water that is bound to the oil droplets cannot be separated with just a FWKO. Heat that is applied to a FWKO is not sufficient to remove enough water to achieve sales specifications. An emulsion treater, by contrast, is required to separate the emulsified oil and water.
Factors in the separation of water from oil are: retention time, heat, chemicals, centrifugal forces, and gravitational forces. Heat and chemicals are two important factors that help remove the last emulsified droplets of water from the oil. A successful emulsion treater should apply the beat evenly to all the incoming emulsion, and then make the emulsion travel the longest possible distance before it exits the emulsion treater. Providing a non-turbulent area and eliminating all incoming gas from the liquids promotes good separation of the oil and water droplets.
In order to provide good retention times many emulsion treaters are made very large. The fluid entering an emulsion treater tends to channel through the vessel if it is not obstructed. A large percentage of the volume in the emulsion treater is not fully utilized unless there are numerous baffles and coalescing plates. These internal baffles and coalescing plates add a large cost to the vessel,
Large diameter vessels can add retention time, however they tend to be expensive and usually limit the pressures to less than 100 psi. Conventional emulsion treaters utilize a complicated system to monitor the oil-water interface, in order to determine when to dump the oil and water. The instrumentation to accomplish this is usually complex and expensive to operate.
Conventional emulsion treaters under consideration fall into two broad categories: horizontal and vertical, each type having its own advantages. Heat distribution is the most critical factor in removing water droplets from the oil. In a horizontal vessel larger firetubes can be added so that more heat can be added to the emulsion. Since a horizonal vessel can add more heat to the emulsion it is preferred by industry. However, much of the heat added is not efficiently applied to the fluid, and consequently heat is wasted in water and is lost up the firetube stack. The most effective design would add minimal heat to the water and have little or no heat go up the firetube stack.